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On last Sunday while I was driving towards one of our relatives place, the car temperature gauge started climbing towards red. And soon "Check Engine" light came on. I was really surprised by this as I never came across any such thing in my last 7 years of ownership.

A couple of minutes later again I cranked the engine and started driving. Hoping that things would be different this time but again the temperature needle started climbing. I somehow managed to reach my sisters place (with couple of stoppages) and took her vehicle (maruti zen) instead. When I returned I checked the coolant bottle and it was nearly dry. I remembered that supervisor at Hyundai workshop once told me to only add water and no coolant if need be. I got myself a bottle of distilled water and filled up the bottle. Drove back to home without any problem. On Monday I checked coolant bottle and found it nearly empty. This time again I put some distill water and went to office. In the evening I purchased Castrol 500 ml coolant and added it to the bottle. The coolant was now well above the full mark.

I reached office and checked the coolant bottle (while the engine was hot). The level was very low (below min mark). There is no sign of any coolant leakage. It seems radiator/vehicle is sucking in whatever I am pouring in the bottle.

Is it normal that coolant gets disappeared from the coolant bottle (without any leakage)? How often should you top up coolant level? What is the approximate quantity of coolant in Santro?

I am planning to take vehicle to Hyundai workshop as I do not think that adding fresh coolant would solve the problem. Meanwhile your input would help me quench my bafflement.

Thanks.

Sometimes there is a small leak from which the coolant can evaporate. In this case you would not see any leakage.
Such leaks are usually found near the joints where the hoses join the reservoir or the radiator.

Since you drove through the heating problem, once the engine is cold, open the radiator cap and check the coolant level, it should be upto the brim.

If not, fill it up and check the level again after your next drive (once engine is cold). If it decreases, there is a leak, if not, the level was initially very low in the radiator, so it kept sucking the coolant from the bottle.

I tried to open the radiator cap (when engine was cold) but it did not open. I could twist the cap and it becomes loose but it won't come off. Is this normal with radiator cap?

If I am able to open the radiator cap then I would be able to assess the coolant level in the car.

Yesterday evening before leaving from the office I topped up the bottle with distill water just one last time and drove back home. There I checked the level and found it OK. Since the engine was hot I decided to re-check the coolant level in the morning. On re-check (this morning) I found coolant level perfectly fine. I drove the vehicle to office and again had a look. To my relief the coolant level is again at right level.

Seems like the coolant level ran low in the radiator before I got the warning lights on and needle soaring. And subseqent refills in coolant bottle were eagerly consumed by raditor to reach to optimum level. Now it appears that radiator had had its full and the coolant bottle is at peace :)

I would keep a watchful eye for next few days to see any signs of inconsistencies. Hopefully things will remain in order.

Thanks everyone for their inputs!

just a basic question --------

I have seen authorised GMI and Hyundai service centres adding water in the coolant bottle.
shouldnt they add coolant rather than water with soaring temperatures and high a/c usage ?

I mean older generation cars could not do without coolant.

If you find that your coolant is disappearing, then you need to do a pressure test on the cooling system. Basically, you use a radiator cap adapter fitted with an air valve and pressurize the system to a bit above the radiator cap rating. For example, if you have a .9 bar cap, then you would pressurize the system to approx. 1.1 bar to test for leaks. You apply air pressure and start looking and listening for coolant or air escaping.

Sometimes, when a radiator or thermostat is replaced, you will have air pockets in the cooling system that take some time to dissipate. This would result in overheating and disappearing coolant. Also, you could also have pin hole leaks in the heater core. A pressure test will usually reveal this.

Occasionally, you run into a failed head gasket which causes a loss of coolant. The coolant could end up in the oil, which you will notice by the milkiness or it might end up in the cylinders where it evaporates out of the engine.

Finally, let me say that water is the best coolant available, specifically distilled water since it does not have minerals in it that can coat the water passages. Antifreeze, or "coolant" as some call it, will reduce the specific heat value of water. It will however give some corrosion protection and also lower the freezing point of the coolant mix. In hot weather, I would run only distilled water, IMHO.

Usually the radiator has a Coolant+ Water mix and the coolant bottle should be pure coolant.

its water that evaporates from the coolant+water mix. so if your coolant doesn't leak then adding water to compensate for reduced level of coolant is right.
coolant does not evaporate.
but if your vehicle leaks coolant then you should add coolant+water.

In the morning prior to starting the engine i checked my coolant level and it was around 60% in the reserve tank. After a drive of 15 minutes i checked it again and it was nearly on the "F" mark on the reserve tank. Which one is correct. Should i put in coolant when the whole thing is cold or when its hot.